1. Field of the Invention
The invention relates to a device for calibrating distance-measuring apparatuses.
2. Description of the Related Art
The distance-measuring apparatuses are commercially available as hand-held measuring apparatuses. Their distance measuring range is a few multiples of 10 m and they are used mainly in building surveying, for example for the 3-dimensional measurement of rooms. The transmitter emits an intensity-modulated beam. In general, wavelengths in the visible range are used, facilitating the aiming at the measuring points. The beam is reflected or scattered by the object to be measured and is picked up by the receiver. The distance from the object to be measured is obtained on the basis of the phase position of the modulated beam relative to the transmitter.
It is known that the accuracy of measurement of these distance-measuring apparatuses is determined to a great extent by environmental influences and apparatus-related influences. For example, varying ambient temperatures, the large dynamic range of the reflection of the illuminated object to be measured, but in particular a component-related temperature drift of the electronics, affect the distance measurement. In order to reduce these influences, a known reference distance in the apparatus itself is used for calibration.
DE 22 29 339 B2 discloses an electro-optical distance-measuring apparatus in which the emitted light beam is modulated in a switchable manner with two different measuring frequencies for rough and precise measurement. In the receiver, the rough measurement frequency is fed directly to the intermediate frequency amplifiers (IF) without mixing. In addition, an auxiliary oscillator whose frequency is chosen so that it corresponds to the difference between the two measuring frequencies is used in the receiver. Thus, the rough measurement frequency and the low frequency which results during the precise measurement after frequency mixing are equal. Consequently, an otherwise usual second auxiliary oscillator is dispensed with, leading to a reduction in the expensive components. When carrying out a distance measurement, the measuring beam is passed alternately over a measuring distance and a calibration distance with the aid of a mechanical switching shutter.
DE 37 10 041 C2 discloses a device for non-contact optoelectronic distance measurement with the aid of fibreoptic bundles. The light at the end of one fibre bundle strikes a reference mirror as reference light while the light of a second fibre bundle is directed as measuring light via a lens to a reflector. The evaluation of the reflected measuring light and reference light is carried out by means of mixing stages which are connected to a common auxiliary oscillator. The mixing stages deliver intermediate frequency signals to the inputs of a phase measurement means.
DE 4 316 348 A1 describes a device for distance measurement, in which an internal reference distance is generated with the aid of a switchable beam deflection means. The beam deflection means is swivelled about an axis under motor power into the measuring light beam path, where it now deflects the measuring light as reference light for calibration to the receiver. As a result of the mechanical switching of the beam deflection means, reference light and measuring light alternately reach the receiver. This switching can be effected several times during the distance measuring process.
During the measuring time, in which the measuring and reference beam are detected in succession, the drift conditions of the electronic components change. All electronic components and lines give rise to signal delays in the signal path of an optical distance-measuring device. These delays are not only of a static nature but also change as a function of time, in particular on the basis of the temperature of the electronic components. In addition to temperature changes of the environment, the self-heating of the electronics, in this case in particular the transmitter electronics, is mainly responsible for the drift of the signals. A phase meter registers these signal delays as phase shifts, which occur in addition to the distance-dependent phase shift actually to be determined.
This effect is particularly pronounced directly after the distance-measuring apparatus has been switched on, since in this state the temperature changes of the electronic components due to their self-heating are the greatest. This results in particularly large signal delays which cause a phase shift of the signals and hence errors in the distance measurement. However, it is precisely battery-operated hand-held measuring apparatuses which are required to measure with the specified accuracy immediately after the apparatus has been switched on. The thermal drift of the electronics is partly compensated by repeated mechanical switching between measuring and reference beam during a measurement. A high accuracy of measurement with short measuring times immediately after switching on the apparatus is however not achieved.
In addition, many apparatuses are set up in such a way that at least the high-frequency electronics of the transmitter automatically switches off after a short waiting time at the end of a distance measurement, since said electronics consume a particularly large amount of electrical energy. As a result of the automatic switching off, the battery of the hand-held measuring apparatus is saved. If a further measurement is required, the apparatus then switches on automatically, the associated thermal drift problems, as described above, being repeated.
The avalanche photodiode usually used as a measuring receiver also makes a further contribution to the accuracy of measurement. Although said photodiode has the advantage of high amplification, it is necessary to accept a high operating voltage dependent on the temperature of the diode. Since, however, the operating voltage has to be adjusted as a function of the diode temperature, the phase position of the signal received and hence the measured value for the distance also inevitably change.
Finally, repeated mechanical switching during the measurement process results in high mechanical stresses and hence considerable wear of the moving parts. Correspondingly complicated designs on the other hand in turn mean high manufacturing costs and generally a large weight and volume.
It is the object of the invention to provide a device for calibration in optoelectronic distance measurement, with the aid of which device high accuracies of distance measurement are achieved in short measuring times and in particular immediately after switching on the apparatus, the reliability of the apparatus is increased and a simple and compact design with low manufacturing costs is permitted.
According to the invention, a part of the high-frequency modulated transmitter beam is always decoupled from the emitter beam of the distance-measuring apparatus and is fed to a reference receiver, e.g. a PIN diode, over an internal reference distance serving as a calibration distance. Said diode is connected to a frequency mixer. This frequency mixer in turn is connected directly to the avalanche photodiode used as a receiver for the measured beam. A high-frequency electrical signal which is to be referred to as the mixer frequency is input into this connection. On the one hand, this mixer frequency is thus mixed by means of the frequency mixer with the high-frequency modulation signal of the reference beam received from the reference receiver, with the result that a low-frequency calibration signal is generated. On the other hand, the mixer frequency is mixed with the high-frequency modulation signal of the measured beam received from the avalanche photodiode, with the result that a low-frequency measuring signal is generated. The avalanche photodiode is a so-called direct mixer. The low-frequency calibration signal and the low-frequency measuring signal are fed to the phase measurement means. Two separate phase meters can be used for simultaneous phase measurement. However, the phase measurement is also possible with only one phase meter, by sequential measurement.
What is decisive is that, by the electrical connection between the frequency mixer coordinated with the reference receiver and the avalanche photodiode, the signal delays which are due to the varying operating voltage of the avalanche photodiode act equally on the low-frequency calibration and measuring signal. Consequently, exactly the same phase shift is produced in the low-frequency calibration and measuring signal and therefore no longer occurs in the phase measurement with subtraction of the measurement and the calibration phase.
Specifically, avalanche photodiodes have about 100 times higher amplification than other photodiodes and hence a correspondingly high sensitivity. For this they require a very much higher and temperature dependent operating voltage in operation. Consequently, avalanche photodiodes must be operated with variable bias voltage dependent on the temperature. As a result of this, the capacitance of an avalanche photodiode changes with the varying bias voltage so that undesired phase shifts are caused. However, these phase shifts are equal both for the low-frequency measuring signal delivered by the avalanche photodiode and for the low-frequency calibration signal, owing to the electrical connection between frequency mixer and avalanche photodiode. Thus, the bias voltage of the avalanche photodiode, which varies as a function of temperature, is eliminated as a source of error for the distance value determined from the phase measurement.
Likewise, the temperature drifts of the transmitter, in particular of the transmitter diode and of the associated driver electronics, are also compensated by the calibration process according to the invention shortly after the apparatus has been switched on. The measuring beam and reference beam are detected simultaneously by constantly supplying a part of the transmitter beam to the reference receiver. This supply can be effected, for example, by decoupling the reference beam from the emitter beam by means of a partly transparent mirror. The decoupled beam passes over a reference distance to the reference detector. A sufficient intensity of the measuring beam leading to the object to be measured can also always be ensured since, with the aid of the present efficient semiconductor lasers as transmitters, the intensity of their emitted beam can be correspondingly regulated.
Because reference and measuring beam are received not in succession but simultaneously and their mutual phase position is measured, a drift of the transmitter in calculating the difference between the phases is eliminated by calibration.
In general, the accuracy of the distance measurement is increased by this optoelectronic calibration, in particular with the requirements that only short measuring times are permitted and that the increased accuracy of measurement is achieved immediately after the apparatus has been switched on. In addition, the measuring times are reduced to about half that of the conventional successive measuring methods, since switching operations are dispensed with. The reliability of the apparatus, too, is improved by the invention since no mechanically movable components are necessary. Moreover, the omission of the mechanical switching device is advantageous for a handheld measuring apparatus, owing to the lower weight and volume. The associated lower manufacturing costs are also advantageous. Finally, the short measuring times permit a substantially larger number of measurements with a given battery charge.